The way that localization works on recent Kindle models (K4 and K5) is pretty standard, actually.
- For "native" (Linux) functionality (minor parts of the Kindle UI, like startup and "USB connected" screens) : gettext (Google: localization mo po)
- For Java (the overwhelming majority of what you interact with): ResourceBundles
- (K5 only, so not relevant for the O.P.) For Javascript (a few things like Wifi selection and browser): I can't remember exactly what library it uses, but it's straightforward as well - just define the translations in the correct file in a subdirectory named according to the locale. More info is buried somewhere inside the thread linked below.
You will find (almost) all the required technical information and tools in
this thread. Yes, it is for a different Kindle model and not really maintained anymore. But as said, the mechanisms are the same on the K4 and K5, and the "abandoned" state is only because nobody is willing to invest effort, not because it's impossible.
So... yes, it's totally feasible, and not terribly difficult, to provide a working "language pack" for any desired language for the K4 or K5 models.
But don't underestimate the effort: it does require
(a) endurance (there are thousands of messages to be translated),
(b) determination (the tools will need to be slightly adapted to account for the different firmware), and
(c) a bit of hacking and/or reverse engineering skills.
If you are willing to show
(a) and
(b), then I will happily help out with
(c) if required. Let me know
